Most of the conventional eye-fundus cameras use a split-image focusing screen to assist in focusing thereof. Please refer to FIG. 1. The split-image focusing screen utilizes two prisms and a slit to form a split image having an upper and a lower rectangular image part on an image sensor, such as a charge coupled device (CCD) sensor. When an eye-fundus camera is not focused, the two rectangular image parts are vertically offset from each other, i.e. they are not vertically aligned with each other. On the other hand, when the eye-fundus camera is focused, the upper and the lower rectangular image part of the split image are aligned with each other to form a complete image.
Since the light intensity acceptable by human eyes is not high, the light source projected onto an examinee's eye via an ophthalmic examination instrument, such as the eye-fundus camera, can have only limited light intensity. Further, as being affected by the physical characteristics of optical elements in the conventional ophthalmic examination instrument, the split image formed on the fundus image captured by the examination instrument is not so clear, as shown in FIG. 2. An examiner can only roughly find two extremely vague rectangular image parts located at a central area of the very dark split image. It is very difficult for the examiner to clearly recognize the positions of the two rectangular image parts and accurately align them with each other to achieve the purpose of focusing the examination instrument.
In view of the inconveniences and drawbacks in focusing the existing ophthalmic examination instrument with the help of a split-image focusing screen, it is desirable to develop an improved image-recognition assisting method to help the examiner quickly and clearly recognize the split image to complete correct focusing of the examination instrument.